Recycling of superalloys with the aid of an alkali metal salt bath

ABSTRACT

The invention relates to a process for recovering valuable metals from a superalloy which has the steps of digesting the superalloy in a salt melt. The salt melt contains 60-95% by weight of NaOH and 5-40% by weight of Na 2 SO 4 .

The present invention relates to a process for the digestion of superalloys, in particular superalloy scrap, in a salt melt and subsequent recovery of the valuable metals.

Superalloys are alloys which have a complex composition, are stable at high temperatures and are based on nickel and cobalt, with additions of other metals, such as, for example, aluminium, chromium, molybdenum, tungsten, tantalum, niobium, manganese, rhenium, platinum, titanium, zirconium and hafnium, and nonmetals, such as boron and/or carbon. The superalloys are high-strength and particularly hard-wearing alloys which are used in motor and engine construction, in energy technology and in aviation and space flight. The particular properties of these alloys are achieved in particular by the addition of rare and noble metals, such as rhenium, tantalum, niobium or even platinum. A good overview of the composition, properties and fields of use of the superalloys is to be found in Ullmann's Encyclopedia of Industrial Chemistry, Volume A13, Fifth Edition, 1989, pages 55-65, and in Kirk-Othmer Encyclopedia of Technology, Volume 12, Forth Edition, pages 417-458.

The superalloys differ from the customary high-melting alloys, e.g. W—Re alloys or Mo—Re alloys, in their particular resistance to oxidation or corrosion. Thus, owing to their excellent oxidation stability, components comprising superalloys are used in the production of blades in aircraft turbines. After elapse of the duration of use, such parts are an important raw material source for recovering rare metals, in particular rhenium, tantalum, niobium, tungsten, molybdenum and platinum.

The recovery of the alloy metals of the superalloys is commercially very interesting owing to the high proportion of expensive metals. Thus, special superalloys contain the metals rhenium in up to 12% by weight, tantalum in up to 12% by weight, niobium in up to 5% by weight and tungsten and molybdenum in up to 12% by weight. Further metals which serve as base metals in the superalloys are nickel and cobalt. For the last-mentioned metals, too, the superalloys are a raw material source from which the recovery of these metals is commercially expedient.

For the recovery of the metallic components from superalloys, a large number of hydrometallurgical or pyrometallurgical and electrochemical processes are known which, owing to their complex embodiments and high energy demand, are not processes which are not carried out on a large scale from commercial points of view, especially owing to the constantly increasing energy prices.

According to the prior art, for the recovery of the metallic components from the superalloys, the latter are melted kept under an inert gas atmosphere and then atomized to give a finely divided powder. In this procedure, a disadvantage is that the superalloys melt only at high temperatures between 1200 and 1500° C. The actual digestion of the superalloy takes place only in a second step by treatment of the powder obtained with acids. Experience has shown that several days are required for this purpose. According to another process, clump-like superalloy scrap is first comminuted by energy-intensive milling processes after prior embrittlement, for example at low temperatures, and then digested by a wet-chemical method at elevated temperatures in mineral acids of a certain concentration and composition, Potter et al., Eff. Technol. Recycling Metal 1971, page 35 et seq.

Furthermore, some processes which envisage the digestion of the superalloy scrap via electrochemical processes are known.

According to U.S. Pat. No. 3,649,487, the high-melting metals present in scraps of an Fe/Ni/Co/Cu base alloy, e.g. tungsten, molybdenum and chromium, are first converted into borides, carbides, nitrides, silicides or phosphides via a melting process by addition of non-metallic compounds of group III, IV or V, melted to give anodes and then subjected to an anodic oxidation. Those metals such as Co, Ni and Cu initially go into solution and are deposited from this at the cathode, while the high-melting metals, remain behind in the anode sludge, for example as borides, carbides, etc. It is disclosed here that the metals Ni, Co, Cu are separated from the high-melting metals, such as W, Mo or chromium, but there is no information at all about whether complete separation of these metals takes place. The document furthermore provides no information about the cost-efficiency of the process.

WO 96/14440 describes a process for the electrochemical digestion of superalloys by anodic oxidation of the alloy in an electrolysis bath with an organic solvent component. The document discloses that up to 10% of water can be added to the electrolyte solution so that the process can still be carried out according to the invention. Otherwise, passivation of the anode occurs through formation of a gel or a firmly adhering oxide layer, which can lead to termination of the electrolysis. The working-up and separation of the valuable substances from the suspension forming as a result of the electrolysis are initially effected by filtration. The filtration residue separated off and containing a part of the alloy metals is then worked up thermally by calcination and subsequently by the customary hydrometallurgical processes.

DE 10155791C1 likewise discloses an electrochemical digestion process for superalloys. In this process, the superalloys are first cast into sheets and then electrolytically digested in an oxygen-free inorganic acid. Here, the problem of anodic passivation is counteracted by reversal of the polarity of the electrodes. The two last-mentioned processes can be implemented economically only under certain general conditions, in particular very high rhenium contents in superalloys.

DE 19521333 C1 discloses a pyrometallurgical digestion of tungsten-containing hard metal and heavy metal scraps. The digestion takes place at temperatures between 800 and 1000° C. in a salt melt which consists of NaOH and Na₂SO₄. In these processes, a sodium tungstate melt is produced, which is dissolved in water after subsequent cooling.

As in the present invention, tungsten hard metal scrap is virtually completely digested there in alkaline, sulphate-containing melt under oxidizing conditions by formation of sodium tungstate. This is not surprising since the metallate is distinguished by high stability and dissolves in the NaOH melt under the reaction conditions. Thus, a complete dissolution process of the hard metal scrap is ensured.

It was an object of this invention to provide a process for the digestion and recycling of superalloys, in particular rhenium-containing superalloy scraps, and working-up for recovery of the valuable materials present therein as a more economical alternative to recycling by anodic oxidation or acid digestion.

The object was achieved by a process for the recovery of valuable metals from superalloys, the superalloys being digested in a salt melt consisting of 60-95% by weight of NaOH and 5-40% by weight of Na₂SO₄ and the melt digestion product formed thereby then being worked up hydrometallurgically with the aim of simple separation of the individual valuable metals.

The digestion is preferably carried out in a salt melt consisting of 65-85% by weight of NaOH and 15-35% by weight of Na₂SO₄, particularly preferably of 70-80% by weight of NaOH and 20-30% by weight of Na₂SO₄.

In the case of superalloys with the digestion of which the present invention is concerned, more than over 50% of the metallic constituents, e.g. nickel or cobalt, do not form metallates under the reaction conditions of DE 19521333 C1, and it was surprising that a corresponding digestion could take place at all. Furthermore, it was surprising that virtually all the nickel and cobalt was present in metallic form after digestion and hence particularly advantageous working-up of the melt digestion product where the use of magnetic separation was possible. At least, this results in a substantial economic advantage over the electrochemical digestion processes cited for superalloys. Superalloys according to the present invention are alloys which contain, as main components, 50 to 80% of nickel, 3 to 15% by weight of at least one or more of the elements cobalt, chromium and optionally aluminium, and 1 to 12% by weight of one or more of the elements rhenium, tantalum, niobium, tungsten, molybdenum, hafnium and platinum.

The process according to the invention is suitable in particular for rhenium-containing superalloys which contain up to 12% by weight of rhenium. The digestion according to the invention of superalloys is advantageously carried out in such a way that up to 10% by weight, preferably up to 8% by weight and particularly preferably up to 5% by weight of sodium carbonate (Na₂CO₃), based on the weight of the salt melt, are added to the salt melt.

Advantageous compositions of the salt melt are listed in Table 1.

TABLE 1 % by weight of % by weight of % by weight of NaOH Na₂SO₄ Na₂CO₃ 85 5 10 80 10 10 70 25 5 80 15 5 75 20 5 72 20 8

The superalloys may be present both in lump form and in pulverulent form (grindings or grinding dusts).

The superalloy digestion can be carried out both in directly heated furnaces, e.g. in furnaces with gas or oil firing, and in indirectly heated furnaces, continuously or batchwise. The furnaces suitable for this purpose are, for example, rotary furnaces and rotary tubular kilns.

The digestion of superalloys is preferably carried out in a moving alkaline melt in a directly fired rotary tubular kiln operated batchwise.

The digestion according to the invention is carried out in such a way that at least 1 kg of salt melt, preferably at least 1.5 kg and particularly preferably at least 2 kg are used per 1 kg of superalloy. In the case of certain superalloys which have rhenium contents greater than 8%, up to 5 kg of salt melt are used per kilogram of superalloy.

The digestion according to the invention of superalloys takes place particularly advantageously with regard to the space-time yield if air and/or oxygen, or a mixture thereof, is passed into the salt melt. A mixture of air and oxygen consisting of 25 to 95% by volume of air and 5 to 75% by volume of oxygen, preferably of 35 to 80% by volume of air and 20 to 65% by volume of oxygen, is preferably passed into the salt melt.

The digestion according to the invention of superalloys is carried out at temperatures of 800 to 1200° C.

Preferably, the digestion is carried out in the temperature range of 850 to 1100° C., particularly preferably at 900 to 1050° C. Good digestion conditions are present if oxidizing agents are additionally introduced into the melt. For example, nitrates, peroxodisulphates, peroxides of the alkali metals and/or mixtures thereof can serve as such. Potassium nitrate, sodium nitrate, sodium peroxide, potassium peroxide, sodium peroxodisulphate, potassium peroxodisulphate and/or mixtures thereof are advantageously used as oxidizing agents. Particularly good digestion rates are achieved if 5 to 25% by weight of the oxidizing component, based on the weight of the melt, are added to the melt.

Advantageous compositions of the salt melt are shown in Table 2.

TABLE 2 % by weight % by weight of % by weight of % by weight of of oxidizing NaOH Na₂SO₄ Na₂CO₃ agent 70 10 — 20 (NaNO₃) 77 5 — 18 (K₂S₂O₈) 80 5 5 10 (Na₂O₂) 60 20 8  6 (NaNO₃)  6 (Na₂S₂O₈) 85 10 —  5 (Na₂O₂)

The melt digestion is particularly advantageously carried out in such a way that a partial oxidation of the superalloy takes place or, after virtually complete oxidation, reducing conditions are established for a certain time. In the digestion process according to the invention, three fractions are pre-formed in the melt itself, consisting of:

-   -   water-soluble alkali metal oxometallates of the metals of the         6th and/or 7th subgroup and/or of the 3rd main group of the         Periodic Table of the Elements and/or mixtures thereof;     -   water-insoluble components from the group consisting of the         metals Co, Ni, Fe, Mn or Cr and/or mixtures thereof,     -   oxides and/or water-insoluble alkali metal oxometallates of the         metals of the 4th or 5th subgroup of the Periodic Table of the         Elements and/or mixtures thereof.

These three fractions are then worked up hydrometallurgically. The present invention therefore relates to a process for working up the superalloy melt digestion product, comprising the following steps:

a) conversion of the melt digestion product into the solid phase by cooling to room temperature, b) commination of the solidified melt digestion product, c) reaction of the comminuted melt digestion product in water at temperatures of less than 80° C. and production of an aqueous suspension containing

-   -   a solution consisting of a mixture of sodium compounds from the         group consisting of NaOH, Na₂SO4, NaAl(OH)₄ and/or Na₂CO₃ and         alkali metallates of the elements of the 6th and/or 7th         subgroups of the Periodic Table of the Elements;     -   a solid metallic phase consisting of the group of metals Co, Ni,         Fe, Mn and Cr;     -   a solid phase consisting of hydroxides and/or hydrated oxides of         the metals of the 3rd main group and of metals of the 4th and/or         5th subgroup of the Periodic Table of the Elements,         d) removal of the aqueous fraction by filtration,         e) separation of the water-insoluble fraction by magnetic         deposition of metallic components,         f) removal of the oxidic fraction.

The process according to the invention is shown schematically in the attached FIG. 1. According to FIG. 1, the superalloy melt digestion product (2) is crushed after cooling to room temperature, then comminuted in a mill and then leached in water. Preferably, the leaching is carried out at temperatures of less than 60° C. and particularly preferably at less than 40° C. The particular feature of the melt digestion comprises the three fractions which are formed therein beforehand and are present during the water leaching as fractions which can be easily separated:

-   -   the filtrate (4) which substantially contains the elements         molybdenum, tungsten and rhenium in the form of their alkali         metallates,     -   the water-insoluble residue (3) which consists of a magnetic         fraction which contains practically the total nickel and cobalt         fractions of the alloy and about ⅓ of the chromium used, in         metallic form, while all other elements are present only as         secondary constituents or in the trace range, and     -   a nonmagnetic fraction (5) which contains the elements         aluminium, chromium, titanium, zirconium, hafnium, niobium and         tantalum in the form of their oxides (e.g. Al2O3, Cr₂O3, TiO2,         ZrO2, HfO2, Ta2O5, Nb2O5), or hydroxides (e.g. Al(OH)3, Cr(OH)3,         Ti(OH)4, Zr(OH)4, Hf(OH)4, Ta(OH)5, Nb(OH)₅ or nitrides (e.g.         AlN, CrN, TiN, HfN, NbN and TaN) or carbides (e.g. AlC, Cr2C3,         TiC, ZrC, HfC, NbC and TaC).

The further working-up of these fractions can be effected by the known methods. Thus, the rhenium can be separated off after the filtration from the filtrate (4) over strongly basic ion exchangers, as described in DE 10155791. The rhenium-free solution containing substantially sodium molybdate and sodium tungstate can be added to the process for obtaining molybdenum and tungsten.

The nonmagnetic residue, which contains up to 15% of tantalum, can be used as raw material in tantalum-metallurgy.

The magnetic residue is advantageously used for the production of cobalt and nickel.

The process according to the invention is suitable in particular for recovering rhenium from superalloys. The present invention furthermore relates to a process for obtaining rhenium from superalloys, comprising the following steps:

a) digestion of superalloys in a salt melt consisting of 60-95% by weight of NaOH and 5-40% by weight of Na₂SO₄, b) cooling of the melt to room temperature, c) commination of the melt digestion product, d) reaction of the comminuted melt digestion product in water at temperatures of less than 80° C. and production of an aqueous suspension containing

-   -   a solution consisting of a mixture of sodium compounds from the         group consisting of NaOH, Na₂SO₄, NaAl(OH)4 and/or Na₂CO₃ and         alkali metallates of the elements of the 6th and/or 7th subgroup         of the Periodic Table of the Elements;     -   a solid metallic phase consisting of the group of metals Co, Ni,         Fe, Mn and Cr;     -   a solid phase consisting of hydroxides and/or hydrated oxides of         the metals of the 3rd main group and of metals of the 4th and/or         5th subgroup of the Periodic Table of the Elements,         e) removal of the aqueous fraction by filtration,         f) removal of the rhenium from the aqueous fraction according to         DE 10155791.

The process according to the invention for obtaining rhenium from superalloys is advantageously carried out in a manner such that up to 10% by weight, preferably up to 8% by weight and particularly preferably up to 5% by weight of sodium carbonate (Na₂CO₃), based on the weight of the salt melt, are added to the salt melt. The removal of the rhenium from the aqueous suspension by means of strongly basic ion exchange resins is preferred.

An advantage of the process according to the invention is that the superalloy digestion in an NaOH—Na₂SO₄ melt is exothermic. By passing in air or an air/oxygen mixture, the process is readily controllable. A further advantage is that the valuable substances can be virtually completely recovered.

The invention is explained in more detail with reference to the following example.

EXAMPLE

1.97 t of superalloy grinding dust (1) were heated together with 2.50 t of NaOH and 0.45 t of Na₂SO₄ to 1110° C. in the course of 4 hours in a rotary furnace directly fired with natural gas and left at this temperature for a further hour. The composition of the superalloy grinding dust is shown in Table 1.

Thereafter, the resulting viscous superalloy melt digestion product was completely poured out of the furnace. The cooled melt was first coarsely crushed and then melted to <2 mm. 5.26 t of pulverulent melt material (2) were obtained, which material was stirred into 7.5 m³ of water for leaching. After the end of the addition, stirring was continued for a further 2 hours, followed by filtration over a filter press and rinsing with 0.5 m³ of water. 2.10 t of filter residue (3) and 9.3 m³ of filtrate (4) were obtained. The filter cake was suspended again in water, and the metallic, magnetic fractions were separated from the oxidic and hydroxidic fractions by circulating the suspension through a magnetic separator by means of a pump. The substantially metal-free suspension was then separated again by means of a filter press, and the filtrates were initially introduced for the next leaching run. 1.46 t of metal sludge (5) and 0.56 t of hydroxide sludge (6) were obtained. The hydroxide sludge (6) was sent to a tantalum facility for recovering the tantalum, and the metal sludge (5) was sent to a nickel facility for further working-up. The rhenium-containing filtrate (3) was passed over ion exchange columns with strongly basic ion exchangers for recovering the rhenium. The further enrichment and purification of the rhenium were effected by standard methods according to the prior art. The rhenium-free outflow of the ion exchange columns was used in a tungsten facility as an initially taken material for the leaching of WO₃. The rhenium yield was 94%.

The composition of the superalloy grinding dust and of the most important intermediates is shown in Table 3.

TABLE 3 % kg % kg % kg g/L kg % kg % kg Al 9.28 183 4.47 235 1.46 30.5 21.9 204 0.12 1.7 5.05 28.4 Co 7.09 140 2.59 136 6.73 141 0.0 0.0 9.46 138 0.37 2.1 Cr 7.17 141 2.62 138 6.69 140 0.0 0.0 3.16 46.2 16.4 92.7 Hf 0.22 4.4 0.08 4.3 0.21 4.3 0.0 0.0 0.09 1.4 0.52 2.9 Mo 1.05 20.6 0.39 20.4 0.01 0.1 2.21 20.5 0.01 0.1 0.0 0.0 Ni 51.3 1001 19.0 999 47.9 1000 0.0 0.0 68.8 1006 3.14 17.7 Re 1.53 30.1 0.58 30.5 0.09 1.9 3.12 29.0 0.13 1.8 0.01 0.0 Ta 4.20 82.8 1.55 81.3 3.93 82.0 0.0 0.0 1.94 28.4 9.55 53.8 Ti 1.53 30.2 0.58 30.5 1.47 30.6 0.0 0.0 0.68 10.0 3.59 20.2 W 4.38 86.2 1.64 86.1 0.04 0.9 9.16 85.3 0.06 0.9 0.0 0.0 Zr 2.33 45.9 0.87 45.5 2.15 45 0.0 0.0 0.97 14.3 5.5 31.0 Non-metallic constituents 9.92 Total of metals 90.08 1775 1807 1476 339 1249 249 

1-21. (canceled)
 22. A process for recovering valuable metals from a superalloy which comprises digesting the superalloy in a salt melt containing 60-95% by weight of NaOH and 5-40% by weight of Na₂SO₄.
 23. The process according to claim 22, which further comprises adding sodium carbonate in an amount not to exceed 10% by weight of the salt melt.
 24. The process according to claim 23, wherein the salt melt contains 75-90% by weight of NaOH, 5-20% by weight of Na₂SO₄ and 5-10% by weight of sodium carbonate.
 25. The process according to claim 22, wherein the superalloy contains one or more of the metals from the group consisting of Ni, Co, Cr or Al as a main component and one or more of the elements from the group consisting of Re, Mo, Ta, Nb, W, Hf or Pt as secondary component.
 26. The process according to claim 25, wherein the superalloy contains 0.5 to 12% by weight of rhenium.
 27. The process according to claim 22, wherein at least 1 kg of the salt melt is used per 1 kg of superalloy.
 28. The process according to claim 22, wherein the digesting is carried out in a moving melt.
 29. The process according to claim 23, wherein the digesting is carried out in a rotary tubular kiln operated batchwise or continuously.
 30. The process according to claim 22, which further comprises passing air and/or oxygen or a mixture thereof into the melt.
 31. The process according to claim 22, which further comprises adding oxidizing component to the melt, wherein the oxidizing component is a nitrate, peroxodisulphate, peroxide of the alkali metal and/or mixtures thereof.
 32. The process according to claim 31, wherein 5 to 25% by weight of the oxidizing component, based on the salt melt, are added to the melt.
 33. The process according to claim 30, wherein the mixture of air and oxygen consisting of 25 to 95% by volume of air and 5 to 75% by volume of oxygen is passed into the melt.
 34. The process according to claim 32, wherein the digesting is carried out at temperatures of 800 to 1200° C.
 35. The process according to claim 32, wherein the superalloy is partly oxidized.
 36. The process according to claim 22, wherein three fractions consisting of: water-soluble alkali metal oxometallate of the metals of the 6th and/or 7th subgroup and/or of the 3rd main group of the Periodic Table of the Elements and/or mixtures thereof; water-insolble components from the group consisting of the metals Co, Ni, Fe, Mn or Cr and/or mixtures thereof, oxide and/or water-insoluble alkali metal oxometallate of the metals of the 4th or 5th subgroup of the Periodic Table of the Elements and/or mixtures thereof are pre-formed in the melt.
 37. A process for recovering valuable metals from a superalloy comprising the following steps: a) converting of the melt digestion product according to claim 36 into the solid phase by cooling to room temperature, b) commination of the solidified melt digestion product, c) reacting of the comminuted melt digestion product in water at temperatures of less than 80° C. and production of an aqueous suspension containing a solution consisting of a mixture of sodium compounds from the group consisting of NaOH, Na₂SO₄, NaAl(OH)₄ and/or Na₂CO₃ and alkali metallates of the elements of the 6th and/or 7th subgroups of the Periodic Table of the Elements; a solid metallic phase consisting of the group of metals Co, Ni, Fe, Mn and Cr; a solid phase consisting of hydroxides and/or hydrated oxides of the metals of the 3rd main group and of metals of the 4th and/or 5th subgroup of the Periodic Table of the Elements, d) removing of the aqueous fraction by filtration, e) separating of the water-insoluble fraction by magnetic deposition of metallic components, and f) removing the oxidic fraction.
 38. The process according to claim 37, wherein the reaction of the melt digestion product in water is carried out at temperatures of less than 60° C.
 39. The process according to claim 37, wherein the reaction of the melt digestion product in water is carried out at temperatures of less than 40° C.
 40. A process for obtaining rhenium from a superalloy consisting of the following steps: a) digesting the superalloy in a salt melt consisting essentially of 60-95% by weight of NaOH and 5-40% by weight of Na₂SO₄, b) cooling of the melt to room temperature, c) commination of the melt digestion product, d) reacting of the comminuted melt digestion product in water at temperatures of less than 80° C. and production of an aqueous suspension containing a solution consisting of a mixture of sodium compounds from the group consisting of NaOH, Na₂SO₄, NaAl(OH)₄ and/or Na₂CO₃ and alkali metallates of the elements of the 6th and/or 7th subgroup of the Periodic Table of the Elements; a solid metallic phase consisting of the group of metals Co, Ni, Fe, Mn and Cr; a solid phase consisting of hydroxides and/or hydrated oxides of the metals of the 3rd main group and of metals of the 4th and/or 5th subgroup of the Periodic Table of the Elements, e) removing of the aqueous fraction by filtration, and f) removing of the rhenium from the aqueous fraction.
 41. The process according to claim 40, which further comprises adding sodium carbonate in an amount not to exceed 10% by weight of the salt melt.
 42. The process according to claim 22, wherein the superalloy is a superalloy scrap. 